Heavy crude oils and residua are characterized by an API (American Petroleum Institute) gravity value of less than 25.degree., usually less than 20.degree., and, when upgraded for the recovery of useful products, are subjected to a so-called "coking" operation, in which volatiles are evaporated off and recovered, leaving a solid carbonaceous solid, known as "coke". The quantity of coke so formed may vary, and is usually in the range of about 10 to 25 wt.% of the oil.
The latter product is potentially useful as a fuel. Unfortunately many heavy crude oils and residua, for example, oil sands bitumen, contain substantial quantities of sulfur, in the form of sulfur compounds, and the coke which results from the coking step also contains substantial quantities of the sulfur. Upon combustion of such coke, substantial quantities of undesirable sulphur dioxide are formed, necessitating elaborate and expensive sulphur dioxide removal equipment if the coke is to be used as a source of thermal energy.